Impedance type humidity sensor with proton-conducting electrolyte

ABSTRACT

A humidity sensing device comprising a solid electrolyte evidencing proton conductivity includes HZr 2  P 3  O 12  or a composite comprising HZr 2  P 3  O 12  /ZrP 2  O 7 . The humidity sensing device is operative over a temperature range from 350°-600° C. The humidity sensor is operated by an impedance-type cell.

RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 08/079,237, filed Jun. 17, 1993, which issued onFeb. 28, 1995 as U.S. Pat. No. 5,393,404, the entire disclosure of whichis incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a humidity sensing device comprising asolid electrolyte. More particularly, the present invention relates toan impedance type humidity sensing device comprising a solid electrolyteevidencing proton conductivity.

2. Description of the Prior Art

Heretofore, the use of solid electrolyte humidity sensors as a means formonitoring or controlling the environment has been limited. Thesedevices typically provide an electrical signal which may bepotentiometric, amperometric or conductometric in nature in response tothe level of humidity in the atmosphere. Among the devices proposed forthis purpose are the galvanic cell type humidity sensors which eitheremploy proton or oxide ion conducting electrolytes as humidity sensingelements. Additionally, impedance type humidity sensors may be employedfor this purpose. The electromotive force evidenced by such cellstypically follows Nernstian behavior which serves as a calibration curvefor the sensor. The proton or oxide ion conducting solid electrolytechosen for use in such devices then becomes the prime factor in theconstruction of such humidity sensors. Workers in the art selectedsintered perovskite-related phases in the barium or strontium cesiumyttrium oxide family (MCe_(1-x) Y_(x) O₃ M═Ba or Sr!) for this purpose.However, studies have revealed that electronic and/or proton ionconduction in these materials results in significant deviations fromNernstian behavior, so imposing additional calibration requirements.Accordingly, workers in the art have focused their interest uponalternative materials in their quest to find humidity sensing propertieswhich will satisfy their needs, particularly those which are operativeat high temperatures in excess of 100° C.

Numerous references disclose gas sensors and humidity sensitive devices.However, none of these references disclose or suggest the specificgalvanic type sensor described herein. Typical of the prior artreferences are the following:

Nakamura et al., U.S. Pat. No. 4,024,036, discloses a protonpermselective solid-state member formed of a heteropoly acid representedby the generic formula, H_(m) X_(x) Y_(y) O_(z) !nH₂ O or a saltthereof. In this formula, X represents at least one member selected fromthe group consisting of boron, aluminum, gallium, silicon, germanium,tin, phosphorous, arsenic, antimony, bismuth, selenium, tellurium,iodine and the first, second and third transition metals, Y representsat least one member selected from the first, second and third transitionmetals, provided that X and Y do not represent the same substance, m, x,y, z and n each represents a positive numerical value. The permselectivemember can be used as an electrolyte in a fuel cell and as a membrane ina hydrogen gas refining system.

Murata, et al., U.S. Pat. No. 4,497,701 discloses a humidity sensitivedevice comprising an insulated substrate, first and second electrodesformed on the surface of the insulating substrate and spaced apart fromeach other, and a humidity sensitive film formed on the surface of theinsulating substrate and covering the surface of the substrate betweenthe electrodes. It includes a conductive powder or a semi-conductivepowder, a solid electrolyte powder and an organic polymer, at least partof which is cross-linked by a zirconium compound, which serves as across-linking agent to form a bridge to the organic polymer and to makethe structure of the humidity sensitive film stable. Additionally, thezirconium compound increases the variation rate of the resistance valueas a function of moisture absorption. Thus, the range of the resistancevalue can be made large and the humidity sensitive device can be used asa dew sensor.

Roy et al., U.S. Pat. No. 4,587,172 discloses a low expansion ceramicmaterial having the molecular formula i(Na)j(Zr_(2-z) Na_(4z)) k(P_(3-x)Na_(x) Si_(x)) O₁₂. This composition evidences a low thermal expansionand may be used in low expansion optical reflective structures. Suchstructures have an optically reflecting film deposited on a ceramicsubstrate having a very small thermal coefficient of expansion.

Yamai, U.S. Pat. No. 4,751,206, discloses a method of making a lowthermal-expansive zirconyl phosphate ceramic, (ZrO)₂ --P₂ O₇. The methodinvolves sintering a fine-powder compact of zinc oxide, magnesium oxide,bismuth oxide, manganese oxide, iron oxide, cobalt oxide, or nickeloxide, at a temperature ranging from 1200° C. to 1700° C. The resultingceramic has a low thermal expansion coefficient.

Yamazoe, et al., U.S. Pat. No. 4,718,991, relates to proton gas sensorsand a method for the use thereof in detecting gasses in oxygencontaining ambients. The described sensor comprises three electrodes, anionization electrode, a reference electrode and a detection electrode,each of which is connected to a proton conductor. Upon short circuitingof the ionization and reference electrodes, a measurement of thedifference of potential across the detection electrode is made, therebyindicating the presence of gas.

Yamai, et al., U.S. Pat. No. 4,751,206, discloses a low thermalexpansion material, potassium zirconium phosphate. This material hashigh strength and high thermal shock resistance. This product may beused for furnace refractories which are subject to thermal shock and asthermal shielding materials such as protective tiles on space vehicleswhich shield the vehicle from the heat of re-entry to the atmosphere.

Kawae, et al., U.S. Pat. No. 4,961,957, discloses an electrochemicalcell having a solid electrolyte body and a plurality of electrodesformed thereon. At least one of the electrodes is porous, for use indetermining the concentration of a subject gas in an atmosphere. Theporous electrode may be comprised of platinum, an alloy of platinum, oranother metal such as nickel, silver, gold, rhodium, palladium, iridiumor ruthenium. The solid electrolyte body used as an oxygen sensor isformed of an oxygen-ion conductive solid electrolyte which includes ZrO₂(zirconia) as a major component, and at least one additive such as Y₂O₃, CaO, Yb₂ O₃, and MgO.

Ammende et al., U.S. Pat. No. 4,976,991, discloses a hydrogen sensorhaving a solid electrolyte comprised of nasicon, titsicon, khibinskite,wadeite or β-Al₂ O₃. The electrodes are formed of platinum, palladium orpalladium oxide.

U.S. Pat. No. 3,276,910 to Grasselli, et al. discloses an ion transfermedium for an electrochemical reaction apparatus for converting chemicalenergy into electrical energy. The invention employs a solid iontransfer medium. The device includes a non-conductive housing, with asolid ion-exchange membrane of a polymeric salt of a Group IV metal inan acid selected from the group consisting of phosphoric and arsenicacids positioned within the housing, electrodes positioned on the solidion exchange membrane, means for introducing a gaseous fuel into contactwith one side of the membrane, and means for introducing an oxidant ontothe other side of the membrane and electrical conductors extending fromthe electrodes to the exterior of the housing.

U.S. Pat. No. 5,133,857 to Alberti, et al. discloses a solid-statesensor for determining the concentration of gases that can react withhydrogen. The device includes a solid-state proton conductor having areference electrode on one side thereof and an electrode which catalysesthe reaction of the gas to be detected. The sensor is connected to apower feed system which supplies a current or voltage impulses. Alsoincluded is a system which detects the value of the potential after eachimpulse. This device can be operated at room temperature.

Alberti, et al. discloses an oxygen conductor, not a proton conductor.The specific composition of this oxygen conductor is described aszirconium hydrogen phosphate or zirconium triphosphate doped withsilicates, such as H₃ Zr₂ PO₄ (--SiO₄)₂ (column 2, lines 57-60).

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to prepare a humidity sensorcapable of operating at elevated temperatures.

A further object of the invention is to provide a galvanic cell typehumidity sensor operative at temperatures in excess of 100° C.

Another object of this invention is to provide a humidity sensor basedupon proton conductivity.

Still another object of this invention is to provide a humidity sensorevidencing high levels of reproducibility and durability.

Another object of the present invention is to provide a protonconducting solid electrolyte appropriate for humidity sensing atrelatively high temperature.

Another object of the invention is to provide a proton conducting solidelectrolyte humidity sensor that is selective (i.e. does not give aresponse when impurity gases such as ethyl alcohol, acetic acid andammonia are present).

It is even another object of the present invention to provide a humiditysensing device based on an impedance type response.

It is yet another object of the present invention to provide animpedance type humidity sensor with applications in automaticallycontrolled systems.

It is even a further object of the present invention to provide ahumidity sensing device with an all-ceramic material "MACOR".

It is still a further object of the present invention to provide ahumidity sensor based on films of functionally gradient materials.

It is an additional object of the present invention to provide ahumidity sensor employing film technology wherein the protonic\oxide ionconductivities of the materials employed can be varied.

It is even an additional object of the present invention to provideminiaturized pH sensing membranes for in-situ pH sensing applications infood processing.

In accordance with the present invention these objectives have beenattained while effectively obviating the limitations of the humiditysensitive devices employed heretofore.

The present invention comprises a humidity sensing device based upon aprotonic Nasicon conductor comprising a galvanic cell based upon a HZr₂P₃ O₁₂ /ZrP₂ O₇ composite humidity sensor operative in the range of350°-600° C. The described sensor evidences Nernstian behavior whichconfines the mechanism of proton conductivity in the compositeelectrolyte sensor.

In another embodiment of the present invention, the humidity sensingdevice comprises an impedance type cell based upon a HZr₂ P₃ O₁₂ /ZrP₂O₇ composite.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood by reference to thefollowing detailed description taken in conjunction with the drawingswherein:

FIG. 1a and FIG. 1b are schematic representations of a galvanic typehumidity sensor of the invention.

FIG. 1c is a front elevational view in cross section of a ceramichumidity probe in accordance with the invention.

FIG. 2 is a graphical representation on coordinates of relativeintensity against 2θ in degrees showing x-ray diffraction patterns of(a) pure (NH₄)Zr₂ P₃ O₁₂, (b) HZr₂ P₃ O₁₂ and the sensor material HZr₂P₃ O₁₂ /Zr₂ P₃ O₇.

FIG. 3 is a graphical representation on coordinates of weight percentagainst temperature in degrees Centigrade showing the DTA and TGA curvesfor the described sensor material.

FIG. 4 is a graphical representation on coordinates of electromotiveforce in millivolts against water vapor pressure, P in mmHg, showing thehumidity dependence of electromotive force of the sensor at 400°, 450°,480°, 500° and 550° C. respectively.

FIG. 5 is a graphical representation on coordinates of electromotiveforce in millivolts against water vapor pressure, P, in mmHg showing thehumidity dependence of electromotive force (EMF) of the sensor of theinvention at 450° C. showing cycling data.

FIG. 6 is a graphical representation on coordinates of EMF in millivoltsagainst the ratio of partial oxygen pressures in the reference andsample compartments showing conduction characteristics of the sensormaterial.

FIG. 7a is a graphical representation on coordinates of temperature indegrees Centigrade against time in seconds showing the temperaturedependence of the response time for the humidity sensor with a change ofpartial pressure of water from 6-8 mmHg.

FIG. 7b is a graphical representation on coordinates of electromotiveforce against time in seconds showing the variation of electromotiveforce with time for the same cell as shown in FIG. 7a with partialpressure changes ranging from 6-12 mmHg and 6 to 18 mmHg at 450° C.

FIG. 8 is a schematic representation of another embodiment of thehumidity of the present invention wherein the sensor is based upon animpedance cell type.

FIG. 9 is a graphical representation of the variation of the compleximpedance of HZr₂ P₃ O₁₂.ZrP₂ O₇ with charge of humidity at 450° C.

FIG. 10 is a graphical representation on coordinates impedance spectrum(log z') against relative humidity showing that conductivity isproportional to humidity.

FIG. 11 is a schematic representation of the set up of the impedancetype humidity sensor of the present invention.

FIGS. 12a and 12b are schematic representations of another embodiment ofan impedance cell type humidity sensor of the present inventionemploying film technology.

DETAILED DESCRIPTION OF THE INVENTION

The first step in the fabrication of the humidity sensor of the presentinvention involves preparing a HZr₂ P₃ O₁₂ /ZrP₂ O₇ composite. Theproton substituted NASICON, NaZr₂ P₃ O₁₂, is obtained by theconventional technique of calcination of (NH₄)Zr₂ P₃ O₁₂, the latterbeing conveniently synthesized hydrothermally in an autoclave lined withpolytetrafluoroethylene. This technique typically involves reacting anaqueous mixture of ZrOCl₂.8H₂ O and NH₄ H₂ PO₄. Crystallization of themixture is then effected under autogenous pressure and the resultantcrystalline product is filtered, washed and dried at ambienttemperature. Thereafter, HZr₂ P₃ O₁₂ is prepared by heating thecrystalline (NH₄)Zr₂ P₃ O₁₂ in air at approximately 650° C. for 5 hours.The other starting material ∝-ZrP(Zr HPO₄ !₂.H₂ O) is synthesized byconventional techniques.

Next, HZr₂ P₃ O₁₂ (HZP) in powdered form is mixed with ∝-Zr(HPO₄)₂.H₂ O(ZrP) in a mole ratio of unity to yield an HZP-ZrP mixture. Theresultant mixture is then ground and pelletized, typically with apressure of 150 klb/in² to yield a dense ceramic pellet. The resultanthumidity sensing element is a sintered compact composite phase of HZr₂P₃ O₁₂ and ZrP₂ O₇ which is mechanically stable.

The resultant pellet is next sintered in air to yield a pellet having adensity greater than 80% of ideal density.

The next step in the fabrication of the inventive humidity sensorinvolves forming electrode connections on the sintered pellet. This endis attained by coating each face of the pellet with platinum ink.Finally, the pellet, bearing platinum electrodes, is heated atapproximately 600° C. for a time period of the order of 10 hours to formthe desired sensor disk.

With reference now to FIG. 1A, there is shown a schematic representationof a galvanic cell 10 in accordance with the invention. Shown in FIG. 1ais a sensor electrolyte 11 comprising a HZr₂ P₃ O₁₂ /ZrP₂ O₇ compositehumidity sensor having platinum electrodes 12 and 13 affixed thereto.Electrolyte 11 is shown disposed within quartz tubing 14 at essentiallythe midpoint thereof and held in place by means of ceramic sealant 15,thereby dividing the cell into two chambers, a reference gas chamber 16and a sample gas chamber 17. In operation, cell 10 is disposed within anelectric furnace 18 and humidity is introduced to reference chamber 16and sample chamber 17 from a suitable water reservoir (not shown) usingair as the carrier gas at a flow rate typically of the order of 220cc/min. For comparative purposes, the humidity in the referencecompartment is fixed at 3.16 mmHg by maintaining the reference waterreservoir in an ice bath. The humidity in the sample compartment isvaried by altering the temperature of the sample water reservoir.

With reference now to FIG. 1b, there is shown a schematic representationof a galvanic cell assembly of the invention. Shown is proton conductingsolid electrolyte 21 disposed in chamber 22 which separates the cellinto reference gas chamber 23 and sample gas chamber 24. When the watervapor in chambers 23 and 24 is different, the following reactions occurat electrodes 25 and 26, respectively:

    H.sub.2 O=2H.sup.+ +1/2O.sub.2 +2e.sup.- (anode)           Equation  1!

    2H.sup.+ +1/2O.sub.2 +2e.sup.- =H.sub.2 O (cathode)        Equation  2!

The equilibrium partial pressure of water in the galvanic cell isexpressed by the Nernst equation:

    E=RT/2F.ln  P.sub.H2O (P.sup.r.sub.O2).sup.1/2 /P.sup.r.sub.H2O (P.sub.O2).sup.1/2 !                                      (Equation  3!

    P.sub.H2O =P.sup.r.sub.H2O.(P.sub.O2/ P.sup.r.sub.O2).sup.1/2 exp (2EF/RT)Equation  4!

wherein P^(r) _(H2O) and P^(r) _(O2) represent the partial pressures ofwater and oxygen, respectively, at the reference electrode 26, E is theelectromotive force of the electrolyte, F is the Faraday constant and Ris the gas constant. Under ambient conditions, P_(O2) is assumed to beequal to P^(r) O₂ and for sensing applications the partial pressure ofwater vapor in the sample gas, P_(H2O) can be estimated from theelectromotive force of the cell in accordance with the followingequations:

    E=RT/2F.ln (P.sub.H2O /P.sup.r.sub.H2O)

    P.sub.H2O =P.sup.r.sub.H2O.exp (2EF/RT)

With reference now to FIG. 1c, there is shown a front elevational viewin cross-section of a ceramic humidity probe in accordance with thepresent invention. Shown is MACOR block 30 having a proton conductingsolid electrolyte sensor 31 comprising a HZr₂ P₃ O₁₂ /ZrP₂ O₇ compositedisposed in MACOR tube 32 and held in place by means of an alumina ring33. Conduits 34 and 35 are used for the introduction of reference andsample gases, respectively, into tube 32. Sensor 31 is connected toplatinum leads 36 and 37. Also shown connected to block 30 are cartridgeheaters 38 and 39. Output meter 40 is connected to signal processor 42which is connected to platinum leads 36 and 37 and to a thermocouple 41which is disposed in block 30.

In operation, block 30 is heated by means of heaters 38 and 39 andhumidity is introduced through the reference and sample gas conduits 34and 35, respectively, from a suitable source. Humidity is monitored bymeans of an output meter 41.

With reference now to FIG. 2, there is shown a graphical representationon coordinates of Intensity against 2θ in degrees comparing x-raydiffraction patterns of (NH₄)Zr₂ P₃ O₁₂, HZr₂ P₃ O₁₂ 0.3H₂ O and theHZr₂ P₃ O₁₂ /ZrP₂ O₇ composite sensor material of the invention. Asnoted in FIG. 2a, hydrothermally synthesized (NH₄)Zr₂ P₃ O₁₂ evidenceshigh crystallinity and a structure identical with that of hightemperature NASICON which evidences two polymorphs which are dependentupon calcination temperature. Below 600° C. a triclinic phase appears,and above 600° C. a rhombedral phase appears, the latter not undergoinga phase transition upon cooling or heating. Both phases respond tochanges in humidity; however, the rhombohedral phase was used as thestarting material for the sensor because of its stability at hightemperature. The x-ray diffraction pattern of the sensor material of theinvention (FIG. 2c) is identical with that of rhombohedral HZr₂ P₃ O₁₂but for a few peaks attributable to ZrP₂ O₇, thereby confirming thematerial as a composite of HZr₂ P₃ O₁₂ and ZrP₂ O₇.

With reference now to FIG. 3, the DTA and TGA curves for a sample of thesensor material of the invention is shown in graphical form oncoordinates of weight against temperature in degrees centigrade. Nophase change is evident over the temperature range (approximately0°-600° C.), so indicating that the material is thermally stable duringa heating/cooling cycle. It is noted that the DTA curve slopes smoothlyduring the entire period of heating, so indicating a gradual endotherm.The TGA heating curve reveals that upon heating a weight loss ofapproximately 0.75% occurs which is attributable to water loss which isabsorbed by the sensor on exposure to air which corresponds with theformulation HZr₂ P₃ O₁₂.ZrP₂ O₇.0.15H₂ O. The TGA cooling curve for thesame sample evidenced a weight gain beginning at approximately 400° C.due to absorption of water at relatively low temperature.

With reference now to FIG. 4, there is shown the electromotive force(EMF) response of the galvanic cell as a function of the log of thepartial pressure of water in the sample compartment at a temperaturewithin the range of 400°-550° C. The Figure reveals that at temperaturesless than 450° C. the EMF values are higher and at temperatures greaterthan 450° C. the EMF values are lower than expected from the Nernstequation and are non-linear. This voltage variation at the lowertemperatures is attributed to absorption of water on the surface of thesensor disk which is confirmed by the TGA cooling curve shown in FIG. 3.

The conduction characteristics of the sensor material were also studiedusing a wet oxygen concentration cell having the same partial pressureof water vapor in both the sample and reference compartments. Thepartial pressures of oxygen in both the sample and referencecompartments were adjusted from 68 to 745 mmHg with helium used as abalancing gas. The measured EMFs at 450° C. of the wet oxygenconcentration cell with changing oxygen concentration shown in FIG. 5follow the theoretical values calculated from Equation 3. This confirmsthat the described sensor operates in the cell by a mechanism conformingto the Equations, 1 and 2.

A similar experiment performed in a wet hydrogen concentration cellevidenced Nernstian behavior as a function of the ratio of hydrogenpartial pressure in the reference and sample compartments (H₂ ^(r) /H₂^(s)). This indicated that under the appropriate wet conditions in bothchambers, the device will operate as an oxygen sensor.

FIG. 6 is a graphical representation on coordinates of electromotiveforce (EMF) in millivolts against the ratio of the partial pressure ofoxygen in the reference and sample compartments. This figure isindicative of the conduction characteristics of the sensor material. Awet oxygen concentration cell with the same partial pressures of watervapor in both the sample and reference compartments was employed.Pressures in these compartments were adjusted with a helium balance gasfrom 68-745 mmHg. At 450°, the measured electromotive forces of the wetoxygen concentration cell with changing oxygen concentration follow thetheoretical values calculated from Equation 3 and noted by reference toFIG. 6, so confirming that the sensor material operates by a mechanismin accordance with Equations 1 and 2.

The response time of the sensor as a function of temperature on changingwater vapor pressure is shown in FIG. 7a. As the humidity varies, theEMF responds rapidly and reaches a steady state within a few seconds atall temperatures studied. At 350° C., the response time was 48 seconds.However, at elevated temperatures, for example 450° C., the responsetime was only 15 seconds, as noted in FIG. 7b.

The effect of selected impurity gases in the water vapor was alsostudied. Thus, for example, ethyl alcohol, acetic acid and ammonia wereintroduced to the system and the EMF of the sensor evaluated. Pure ethylalcohol, acetic acid and ammonia, respectively, were mixed with water ina volume ratio of 100 ppm or 1000 pm. The solution mixture served as thesource of saturated water vapor plus impurity vapor supplied to thesample compartment. The sensor material was found to be stable withrespect to each of the impurity gasses and humidity sensing was notaffected within the experimental error of measurement for 100 ppmimpurity gas concentration. This evidences the selectivity of thesensor. As the concentration of impurity gas increased beyond 100 ppm,the EMF value also increased. At this temperature, the EMF appears to bedependent upon the ethyl alcohol concentration at values greater than100 ppm. This phenomenon is attributed to proton reactivity of the ethylalcohol molecules which are absorbed on the surface of the sensor diskin the sample compartment which provides additional protons and enhancedEMF, so suggesting the use of the humidity sensor as a proton-containinggas sensor and/or catalyst.

In another embodiment of the present invention, the humidity sensor maybe based on an impedance cell type. The humidity sensor of thisembodiment is based on impedance type response to changing humidityconditions of a proton conducting solid electrolyte.

The impedance type humidity sensor is also based on a NASICON compositematerial, HZr₂ P₃ O12-ZRP₂ O₇. This device is designed for sensinghumidity at temperatures exceeding 100° C. The device may be fabricatedfrom an all ceramic composite MACOR.

As shown in FIG. 8, the sensor 131 is connected to platinum electrodes136 and 137 which are interconnected with electrode leads 134 and 135which run from the electrodes 136 and 137 through conduits 132 and 133.The sensor 131 and electrodes 136 and 137 are disposed within furnacecore 138 of furnace 139 which includes insulation 140 and which issupported by legs 141. The furnace 139, like the cartridge heatersreferred to in respect to the galvanic cell type sensor, heats thesensor to its operating temperature. A thermocouple 142 extends to thesensor 131 and a sample gas is introduced to the sensor through conduit143. This embodiment of an impedance type cell is only but one exampleof an impedance based humidity sensor, and other configurations of animpedance type cell are considered within the scope of the presentinvention.

The results obtained with this composite are reproducible for humiditysensing at temperatures in the range of 200°-500° C. in both impedanceand e.m.f. modes of operation. This device provides excellent responsetime to changing humidity conditions. However, during prolongedoperation at 450° C., the metal covered cartridge heating elements mayreact with the ceramic material into which they are housed, resulting ina breakdown of the sensor operation primarily due to the un-matchedthermal expansion coefficients of the metal and the ceramic. Inaddition, a need for a reference gas in the operation of e.m.f. typedesign was a concern.

In this impedance-type humidity sensing device, the sensor is heateduniformly with an external furnace. This embodiment, unlike the previousEMF-type, is based on the humidity-dependant impedance of the sensor andtherefore does not require reference humidity for its operation. Theimpedance type sensing characteristics of the HZr₂ P₃ O12-ZRP₂ O₇composite have been previously established herein.

The sensors developed for impedance type response are proton conductingsolid electrolytes.

The operative device may be integrated with custom designed electroniccircuitry and software installed on a computer, which is used in themeasurement of the impedance of the sensor.

In view of problems such as excessive local heating and corrosive attackof the cartridge type contact heaters on the MACOR, it is desirable tohave an external furnace assembled with heating elements to provideuniform heating in the sensing area. The temperature of the furnace iscontrolled with a thermocouple positioned at the sensor.

In this embodiment of the device, the sensor element can be maintainedat 450° C. in a cavity drilled into the MACOR block. The heating of theceramic block is achieved by means of cartridge type heating elements.

The all-ceramic construction significantly minimizes the heat transferbetween the furnace and processing ends of the device. Accordingly,significant improvement in the size of the device has been achieved andthe water cooled jacket near the signal-processing end was eliminated.

Another advantage of the new design is that the sensor element can beassembled separately by sandwiching the composite pellet between a pairof alumina o-rings using a high temperature ceramic adhesive. Thusincorporation of a new sensor element into the device is fairly easyminimizing the down-time required for maintenance.

The stability of the prototype for continuous operation at approximately450° C. has been excellent, however, the device must be heated gently toavoid mechanical failure initiated by thermal shock.

Humidity dependant impedance characteristics of the sensor pellets wereevaluated by applying a low frequency a.c. signal (10 Hz) and measuringthe resulting response as function of moisture, while the sensor ismaintained at 400°-450° C.

The impedance characteristics of the HZr₂ P₃ O12-ZRP₂ O₇ composite as afunction of humidity have been evaluated earlier. FIG. 9 shows that thecomplex impedance of the composite at 450° C. decreases dramaticallywith increasing relative humidity.

It is evident from FIG. 10 that the real part of impedance as a functionof humidity is frequency-independent in the low frequency region (10-20Hz). This is advantageous for device application, because it requiresthe use of only a single low frequency for the measurement of impedanceas a function of humidity. The a.c. conductivity (S), measured byimpedance spectrum (log z'), is directly proportional to the humidityand is reproducible for each sample studied. The relationship between Sand the relative humidity (RH) is:

    s=A·a.sup.(RH/100) ; where A and a are constants. Eq. 5!

The schematics of the measuring system are shown in FIG. 11. A Tektronixfunction generator may be used to apply a constant frequency of 12 Hzacross the sensor. The signal is fed through a built-in Faraday boxwhich is interfaced with a computer and the response in the form ofphase shift can be recorded at different humidity and temperatureconditions. Thus, the frequency generated by the generator is comparedwith frequency from the sensor. The measured impedance is related to therelative humidity according to Equation 5!.

Another embodiment of HZr₂ (PO₄)₃.ZrP₂ O₇ composite as a humidity sensoris shown in FIG. 12. Again, the humidity sensor is based on a humiditysensitive material, for example HZr₂ (PO₄)₃.ZrP₂ O₇. The device is madeby first screen printing two electrode contacts 212 and 213 on analumina or other substrate 214 and then screen printing the humiditysensitive film 211 (e.g. HZr₂ (PO₄)₃ or HZr₂ (PO4)₃.ZrP₂ O₇ composite)thereover (FIG. 12b). On the other side of the substrate 214 is theheater 218, which is made by printing thick film heater 218 of Pt or Agor other desired material as shown in FIG. 12a. The heater isessentially a resistance wire attached to a power supply to providecurrent to heat the sensor to approximately 450° C. This type ofconstruction permits the sensor to be constructed to a very small size(less than one square inch area).

Pastes of conducting metals, such as platinum and silver, or some ionicconductors are commercially available. There are three main componentsin these pastes: the active materials (metal, ceramic or ionicconductor), the fluxing agents (glass frit and bismuth oxide) and theorganic solvents and binders. Composition can be varied to meet specificproperty requirements (viscosity, thermal expansion, grain size, surfaceproperties).

A thin or thick humidity sensitive film comprising HZr₂ P₃ O₁₂ or HZr₂P₃ O₁₂.ZrP₂ O₇ may be printed on the substrate over the electrodecontacts. The film may be prepared from a sol-gel solution. The film maybe printed, screen printed, spin-coated or applied onto the substrate inany manner known in the art. The film could range from a thickness of afew angstroms to many thousands of angstroms. Alternatively, the filmcould be prepared as a paste, in a similar manner to the preparation ofthe pellets hereinbefore discussed, and then painted onto the substrate.

The platinum electrodes are hooked up to the output, and the impedanceis measured. The circuit is completed by the humidity sensitive filmextending between the electrodes on the substrate. The impedance gives adirect relationship to humidity, which is read and interpreted by acomputer. Like the previous embodiment, the sensor of this embodiment isinterconnected with a computer and a function generator as shown in FIG.11.

While the invention has been described in detail in the foregoingspecification and the exemplary embodiments have been alluded to forpurposes of illustration, it will be understood by those skilled in theart that such has been solely for purposes of exposition only and arenot to be construed as limiting.

What is claimed is:
 1. An impedance cell humidity sensor comprising:asubstrate; heating means affixed to one side of the substrate; a pair ofspaced apart electrodes affixed to the other side of the substrate; afilm of humidity sensitive material comprising a composite of HZr₂ P₃O₁₂ and ZrP₂ O₇ applied over and interconnecting the electrodes; and afunction generator for applying a low frequency across the sensor. 2.The sensor of claim 1 wherein the low frequency applied across thesensor is in the range of 10-20 Hz.
 3. The sensor of claim 1 wherein thelow frequency applied across the sensor is approximately 12 Hz.
 4. Thesensor of claim 3 wherein the sensor operates in the temperature rangeof 350° C. and 600° C.
 5. The sensor of claim 3 wherein the sensoroperates at approximately 450° C.
 6. The sensor of claim 1 wherein theelectrodes are applied to the substrate by screen printing theelectrodes thereon.
 7. The sensor of claim 6 wherein the film ofhumidity sensitive material is applied to the substrate and theelectrodes by screen printing the film thereon.
 8. The sensor of claim 7wherein the heating means comprises a resistance wire attached to thesubstrate.
 9. The sensor of claim 8 wherein the heating means is appliedto the substrate by printing the resistance wire on the substrate.